Well. There's a lot to discuss for the background.
First of all, you need to decide how important is your background to your story. In most stories, background is tremendously important. But for some it isn't. If your story is more about the conversation and humour (like gag, slice of life, etc), then you could have a very simple color background or whatever.
Next, you need to decide what kind of style is needed. Do you want your story to be real and relatable? THen perhaps realism is required for your background. Is yours more cute imaginary? Then perhaps simple colored drawings.
And I could go on about this.... Maybe I will write an article on this later, but Im just gonna be realistic and help you with that particular image.
The main thing you need to know is, farther things are, blurry they become. Clsoer they are, more detailed, crisp they are. Also farther things are, they tend to become grey (what kind of grey depends on the time/setting of the pic). Also remember to make your drawing style of the background to match that of your character.